Eleven patients with dextro transposition of the great vessels (TGV) and subpulmonic (left ventricular outflow) obstruction documented by cardiac catheterization and angiography were studied by echocardiography. Echocardiographic features which characterized the outflow obstruction included prolonged diastolic apposition of the mitral valve with the ventricular septum (10 patients), smaller width of the pulmonary artery as compared to the aortic root (10 cases), and narrowing of the subpulmonic area demonstrated by beam angulation studies through the planes of the mitral valve and the pulmonary artery (five cases). In addition, 4 patients showed abnormal systolic anterior movements (SAMs) of the anterior mitral leaflet resembling those observed in idiopathic hypertrophic subaortic stenosis. The SAMs were large in three patients and may have accentuated the outflow obstruction. Additional evidence for this was provided by the movement of the pulmonary valve towards closure with the onset of the mitral SAMs. These features were uncommon or absent in a comparison group of 17 patients with TGV and no outflow obstruction. Echocardiography appears to be useful in the diagnosis of subpulmonic muscular obstruction in TGV. Demonstration of definite mitral SAMs in some patients suggests the presence of associated functional obstruction of the type seen in idiopathic hypertrophic subaortic stenosis.